Step 2: set up a Technitium container in host networking mode
Step 3: configure Technitium with a stub zone pointing your ts.net name at 100.100.100.100
Step 4: set up a zone for whatever.tld
Step 5: set up a DNAME record for ts.whatever.tld pointing at your ts.net domain
Result: querying this new DNS server with machine.ts.whatever.tld resolves to machine.blah-foo.ts.net resolves to that machine's 100.64.0.0 address.
My point was that MagicDNS is implemented in the Tailscale client on each machine (fault tolerant, 0ms latency) and has almost all the things necessary (DNS resolver, push mechanism for record updates) except for a custom defined zone.
Running `drill @100.100.100.100 <node_name>.<magic_dns_domain>.ts.net` is 0ms because it's local, and doesn't depend on a single DNS server running somewhere on my Tailscale network.
TBH I find Docker networking a struggle and usually disable the `iptables` stuff and end up configuring my own rules. Painful, but at least less intrusive.
On the note of Tailscale+Docker networking, gluetun[0] is pretty awesome. It runs a Wireguard (not tailscale compatible, yet) instance within a Docker container and then you share that networking namespace with the other containers effectively confining them to the VPN. Comes with basic container namespace firewall configuration and DNS over TLS configuration.